hi all:
I'm read the source code of RPS and RFS, I think the key function of RPS and RFS is get_rps_cpu(), and
It's codes is:
###############################################################
3079 static int get_rps_cpu(struct net_device *dev, struct sk_buff *skb,
3080 struct rps_dev_flow **rflowp)
3081 {
3082 struct netdev_rx_queue *rxqueue;
3083 struct rps_map *map;
3084 struct rps_dev_flow_table *flow_table;
3085 struct rps_sock_flow_table *sock_flow_table;
3086 int cpu = -1;
3087 u16 tcpu;
3088 u32 hash;
3089
3090 if (skb_rx_queue_recorded(skb)) {
3091 u16 index = skb_get_rx_queue(skb);
3092 if (unlikely(index >= dev->real_num_rx_queues)) {
3093 WARN_ONCE(dev->real_num_rx_queues > 1,
3094 "%s received packet on queue %u, but number "
3095 "of RX queues is %u\n",
3096 dev->name, index, dev->real_num_rx_queues);
3097 goto done;
3098 }
3099 rxqueue = dev->_rx + index;
3100 } else
3101 rxqueue = dev->_rx;
3102
3103 map = rcu_dereference(rxqueue->rps_map);
3104 if (map) {
3105 if (map->len == 1 &&
3106 !rcu_access_pointer(rxqueue->rps_flow_table)) {
3107 tcpu = map->cpus[0];
3108 if (cpu_online(tcpu))
3109 cpu = tcpu;
3110 goto done;
3111 }
3112 } else if (!rcu_access_pointer(rxqueue->rps_flow_table)) {
3113 goto done;
3114 }
3115
3116 skb_reset_network_header(skb);
3117 hash = skb_get_hash(skb);
3118 if (!hash)
3119 goto done;
3120
3121 flow_table = rcu_dereference(rxqueue->rps_flow_table);
3122 sock_flow_table = rcu_dereference(rps_sock_flow_table);
3123 if (flow_table && sock_flow_table) {
3124 u16 next_cpu;
3125 struct rps_dev_flow *rflow;
3126
3127 rflow = &flow_table->flows[hash & flow_table->mask];
3128 tcpu = rflow->cpu;
3129
3130 next_cpu = sock_flow_table->ents[hash & sock_flow_table->mask];
3131
3132 /*
3133 * If the desired CPU (where last recvmsg was done) is
3134 * different from current CPU (one in the rx-queue flow
3135 * table entry), switch if one of the following holds:
3136 * - Current CPU is unset (equal to RPS_NO_CPU).
3137 * - Current CPU is offline.
3138 * - The current CPU's queue tail has advanced beyond the
3139 * last packet that was enqueued using this table entry.
3140 * This guarantees that all previous packets for the flow
3141 * have been dequeued, thus preserving in order delivery.
3142 */
3143 if (unlikely(tcpu != next_cpu) &&
3144 (tcpu == RPS_NO_CPU || !cpu_online(tcpu) ||
3145 ((int)(per_cpu(softnet_data, tcpu).input_queue_head -
3146 rflow->last_qtail)) >= 0)) {
3147 tcpu = next_cpu;
3148 rflow = set_rps_cpu(dev, skb, rflow, next_cpu);
3149 }
3150
3151 if (tcpu != RPS_NO_CPU && cpu_online(tcpu)) {
3152 *rflowp = rflow;
3153 cpu = tcpu;
3154 goto done;
3155 }
3156 }
3157
3158 if (map) {
3159 tcpu = map->cpus[reciprocal_scale(hash, map->len)];
3160 if (cpu_online(tcpu)) {
3161 cpu = tcpu;
3162 goto done;
3163 }
3164 }
3165
3166 done:
3167 return cpu;
3168 }
###############################################################
I know the RPS/RFS get the CPU id by this function.I think RPS can't work,
when RFS is active, beacuse the implementation of RPS is between 3158 and 3163.
and the codes between 3123 and 3156 are used for RFS.
I think RPS is work just in this two conditions:
1. When the packet first arrived this host, the value of tcpu and next_cpu are RPS_NO_CPU.
2. When the destination of packet is not this host, the value of tcpu and next_cpu always are RPS_NO_CPU.
My question is: If the RFS is enable, the RPS can't work?
Thank you.
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